There's a sunny, if not particularly warm, week ahead here in California. Perhaps it might be a nice time to get out of the house, or to go to the park, maybe even feed some birds. But hold off on that third option: A new study shows the food you hand out to birds may be upending the natural balance of urban bird communities.

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Bird feeding might conjure up images of old ladies tossing out bits of bread for Central Park pigeons to gobble up, but actually it's big business. Americans buy something like one trillion pounds of bird seed a year, and both the Cornell Lab of Ornithology and the Audubon Society provide tips and information to make feeding the birds easier—the Lab of Ornithology even suggests feeding can make life easier for the birds, particularly in the resource-scarce winter.

But there could be a downside to feeding the birds at such a large scale, according to doctoral student Josie Galbraith and her advisors at the University of Auckland in New Zealand and Griffith University in Nathan, Australia.

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The invading species' have rather broader palettes—they'll pretty much eat anything; native New Zealand birds consume only nectar or insects.

Though researchers and governments both promote the practice in the United States and abroad, "bird feeding is essentially a massive global supplementary feeding experiment, yet few studies have attempted to explore its ecological effects," Galbraith and her team write in Proceedings of the National Academy of Sciences. As feeding birds becomes more and more popular, the researchers wondered whether the hobby might have broader impacts on birds' ecosystems.

To find out, they set up 11 volunteers in Auckland with bird feeders for their gardens and supplies of bread crumbs and seed. Over the next 18 months, the team counted the number and species of birds that showed up at the gardens over the course of a 10-minute interval. To put those counts in context, they compared them with similar data collected from 12 control sites in Auckland residential gardens that hadn't been outfitted with feeders.

As one might expect, the feeders attracted many more birds than the non-feeding control sites, but that increase was almost entirely due to non-native bird species such as house sparrows and spotted doves. The team counted nearly the same number of native birds in both control and test gardens—typically seven or eight during a 10-minute stretch, representing native species such as the grey warbler or Tūī. In feeder gardens, however, they counted roughly 30 individual non-native birds, twice the number observed at control sites. Likely, that's because the invading species' have rather broader palettes—they'll pretty much eat anything; native New Zealand birds consume only nectar or insects.

That could spell trouble for native birds, the team writes. Grey warbler populations around the feeders actually declined during the experiment, suggesting that the large population of invading species might force foraging warblers out of the gardens.

Quick Studies is an award-winning series that sheds light on new research and discoveries that change the way we look at the world.